Dual discharge tube excimer lasers are used in a variety of applications to include spectroscopy, combustion diagnostics, harmonic generation, stimulated Raman scattering, Brillouin scattering, laser fusion and remote sensing. A standard optical set-up for a dual discharge tube excimer laser is shown schematically in FIG. 1 and referenced generally by numeral 10.
Excimer laser 10 has first and second discharge tubes 12 and 14, respectively. Discharge tubes 12 and 14 are filled with gas mixtures such as argon fluoride, krypton fluoride, xenon chloride, xenon fluoride, etc., as is known in the art of excimer lasers. The oscillator portion of excimer laser 10 includes: discharge tube 12; variable circular aperture elements 16 and 18 disposed at either end of discharge tube 12; optical prisms 20, 22 and 24; an optical grating 26; and an output coupler 30.
As is known in the art, when discharge tube 12 is energized (i.e., its electrodes are energized), a gaseous lasing medium is generated in discharge tube 12 and transmitted therefrom as a laser beam. The laser beam passes through aperture element 18 and impinges on the front surface of each of prisms 20/22/24 at Brewster's angle before impinging on optical grating 26. The cross-sectional shape of the lasing medium and resulting laser beam is defined by discharge tube 12. The bandwidth of the laser beam is selected as the beam is transmitted back through prisms 20/22/24 and aperture element 18 before re-entering discharge tube 12. The bandwidth-defined laser beam exits the opposite side of discharge tube 12 and passes through aperture element 16 before impinging upon output coupler 30 which is typically a partially reflective mirror. Output coupler 30 outputs a laser beam 32 which is turned 180.degree. by mirrors 34 and 36.
Laser beam 32 is amplified by a combination of a concave mirror 38, discharge tube 14 and a window mirror 40 where mirrors 38 and 40 form an unstable resonator. Specifically, laser beam 32 first passes through a small hole (not shown) in concave mirror 38 and discharge tube 14 before impinging on window mirror 40. Mirror 40 has a flat window portion 40A with a meniscus lens 40B at its center. Flat window portion 40A is slightly reflective (e.g., 10%) at the wavelength(s) of interest while meniscus lens 40B is highly reflective (e.g., 95%) at its center. As a result of this structure, the laser beam passes back through discharge tube 14 along a laser beam path 32A. Laser beam path 32A exits discharge tube 14, reflects off concave mirror 38, and is directed back through discharge tube 14 along a laser beam path 32B which exits flat window portion 40A of mirror 40 as the output laser beam. That is, three passes through discharge tube 14 are used to amplify the laser beam.
While the above-described excimer laser design performs satisfactorily, improvements in terms of decreased divergence, increased output energy, locking efficiency, range of tunability and spectral brightness are goals that are continually sought.